Carl Linnaeus’s birthday is 23 May, so each May I think of him and his curious legacy. Revered and derided, contemporary and anachronistic, Linnaeus lived seven decades, and managed to steer the world of botany in irrevocable directions by the time he was 45. Born in 1707 in northern Sweden, he took to plant study as a young man, eventually attending the university at Uppsala as a student in botany and medicine. By Linnaeus’s arrival in Uppsala, the newest, cutting edge realizations that plants reproduce sexually (with pollen cast as male and the pistils with their ovules interpreted as female) had begun to shape the core philosophy baked into his career path.

Though the work of uncovering sexual reproduction in plants was produced by Camerarius and many other botanists, Sebastien Vaillant gains credit for his dramatic publicizing of this reproductive information through the single lecture he delivered in 1717, here published in 1718*

Today, Linnaeus and his understanding of plants are old news, mostly regarded as fundamental fossils. But plant names are important links, and Linnaeus remains significant because he systematically brought every available plant into compliance with a simple way of clustering and naming. In the Linnaean conventions we inherit, different kinds of plants (we call these species) fall out as examples of general groupings we call genera. White oak, Quercus alba, and Coast Live Oak, Quercus robur, are two different species in the genus Quercus.

The page from Linnaeus’s Species Plantarum (1753) in which he describes Quercus alba (#10) and Quercus robur (#12). Note the marginal disposition of the species (his specific epithet)…

The simplicity and thoroughness with which Linnaeus applied his way of naming plants proved immediately practical, sweeping away Europe’s awkward and forgettable hodgepodge of naming conventions. Today, by international agreement (The International Code of Botanical Nomenclature…), botanists throughout the world base all plant names on the format utilized in Linnaeus’ 1753 Species Plantarum. One result of those rules is the ever-present “L.” moniker, which is attached to thousands of plant names Linnaeus recognized.

But the naming convention was simply a practical device for Linnaeus; categorization was his real goal. For contemporaries, many of whom idolized him, the immediate value of Linnaeus’ publications was the way he organized plant genera. It was a straight-forward method, a system easily memorized and utilized. For the first time, anyone could examine a new plant and know where to find it, or file it away – that is, how to classify it so someone else could know where the name and information about a plant might have been stashed.

Georg Dionysius Ehret created this simple illustration for Linnaeus, diagraming the basic arrangement of anthers for each of the 24 classes of plants. Note that in his lettering, Ehret does not include today’s letters “J” and “W”

This system emerged because Linnaeus was among the first generation of botanists aware that seed are the fallout of sexual reproduction; he took that fresh concept and ran with it. The newly obvious reality that sexual reproduction was crucial to the nature of every species meant Linnaeus interpreted characteristics of reproductive organs as directly correlated with the essence (therefore the definition) of each different kind of plant. The result was his sexual system of classification – a legend in its time.

Linnaeus grouped plant genera (and therefore the species assigned to each genus) into Classes based on numbers and character of stamens, and within the Classes, into Orders based on the numbers and characteristics of pistils or further information on stamens. It was straightforward – ten stamens meant a plant was in Decandria while five stamens relegated a plant to Pentandria.

On page 302 of Species Plantarum, Linnaeus includes the genus Lilium in the class Hexandria, because each flower has six stamens. Having a single pistil (even though it is 3-carpellate), Lilium falls out as order Monogynia.

The popularity of his sexual system of classifying plants was short-lived however. Within just a few years of Linnaeus’s death in 1778, botanists published systems that grouped genera into what seemed to be more natural families – reflecting affinities suggested by many characteristics, and accepting the fact that in some genera the number of anthers is not a significant difference. A century later, the obvious affinities would be interpreted as ancestral and evolutionary in origin.  Though his classification crashed and burned quickly, we remain dedicated to Linnaeus’s residual legacy, our functional binomial system of nomenclature.

But not so fast.  Don’t completely discard all of that sexual classification stuff. During the 2007 celebration of Linnaeus’s 300th birthday, I decided it would be interesting to attempt organizing a few study sessions replicating the experience people would have had with Linnaeus’s methods. He was a great and popular teacher – hundreds of people attended his lectures. There must have been something useful there.

With printed copies of his simple system, the students and I worked through many plant samples (staying with the European material Linnaeus knew best). What a revelation!

Linnaeus’s simple Latin key to his Classes of the Vegetable Kingdom. See the figure below for a translation to English.
The English translation of Linnaeus’s key. Note he refers to flowers as nuptial beds, and the combination of male (stamens) and female (pistils) as a marriage.

Of course there was no way we discovered that Linnaeus’s sexual system has much to say about how plants should be grouped or classified  But we did learn how quickly his system cut through the mysteries of unknown plants. Using the sexual system to guide study and identification, we stepped right into Linnaeus’s times and challenges. His system worked, quickly and intelligibly, getting us right to the correct genera – which was his goal. Following his methods proved to be a great teaching method, bringing novices effortlessly into an appreciation of plant structure and diversity. Working through flowers from Linnaeus’s perspective is wonderfully enlightening, engaging, and worthwhile. No wonder he and his students loved this method. Linnaeus studied thousands of different kinds of plants; the sexual system represents the life-experience of the most brilliant field-botanists who ever lived.

Flowers of Lilium are easily determined as class Hexandria and order Monogynia… See individual flower below. (Lilium ‘Siberica’ at Longwood)
Lilium at Longwood – note the easily spotted six stamens

Of course, the system has serious limits. It gets you to a place in a list or chart, but it doesn’t reckon on today’s reality – the sheer number of different kinds of plants we have come to understand there are on Earth. Linnaeus’s reckoning suggested a matrix of possible combinations, predicting on-going discovery of plants would yield less than 50,000 kinds. He knew (or at least he named) 7700 different species, and thought all the world’s plants would be known in short order. That didn’t happen. Today we recognize over 300,000 accepted species, and continue to add new kinds. So honestly, there is nothing that Linnaeus’s system brings to the table in regard to contemporary understanding of plant affinities or evolution.

But for students who want to learn more plants, and how to identify them, Linnaeus’s methods have much to offer. Following his system gives access to Linnaeus’s approach to making sense of the great range of plant diversity – an approach molded through experience and a genius for comprehending and organizing the breadth of creation.

Modern botanists regard Linnaeus’ categorizing system as ancient history, and even his legacy of naming plants by a genus and species (binomial nomenclature) has become annoying to many contemporary systematists. It’s a practical way of naming that’s unlikely to go away. But binomial nomenclature reeks of assumptions that different kinds of plants play by the same rules, suggesting an impossible perfection in the strategies plant populations employ to function and maintain themselves in nature. That’s an unrealistic viewpoint; plants exhibit a wide range of biologies. No one keeps a logbook nor charts a course; evolution constantly erases its tracks, and has no pre-ordained direction.

Today we comprehend plant biodiversity as showing natural affinities among plants, affinities that are explained from an evolutionary viewpoint. A natural system (an outline) of the plant kingdom attempts to align formal taxonomic categories (genus, family, order) as reflections of branches (which we call “clades”) in the tree of life. But it’s an imaginary tree. Scientists cannot truly agree how to define species, and nobody will ever know enough to flesh out missing parts of the “tree”. Nature is not complicit in human attempts to pigeon-hole the entirety of creation; as Harold Bold often said: “Nature mocks at human categories.”

Wedgewood portrait medallion of Linnaeus

*see Paul Bernasconi and Lincoln Taiz, transl., 2o02. “Sebastian Vaillant’s 1717 lecture on the structure and function of flowers,” Huntia 11(2): 99-118.

Post Script

Some of Linnaeus’s most ardent supporters were adamant in their adherence to his system. Among the most colorful was Constantine Rafinesque, who wrote in his Flora Telluriana: “…whoever unities Azalea to Rhododendron sins against Linnaeus and Nature!” Today, botanists recognize Azaleas as Rhododendrons that have five rather than ten stamens. But to Linnaeus (and Rafinesque), they constituted different genera in distinct classes (Pentandria and Decandria, respectively). See illustrations below.

Placement of Azalea in the Species Plantarum class Pentandria, tucked in with Plumbago and Phlox.
Rhododendron ‘Susan Hill’ – a hybrid Azalea with 5-6 stamens.
Rhododendron, categorized in Decandria along with other plants producing ten stamens
A hybrid Rhododendron, showing ten stamens

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